Extraterrestrial life, also called alien life or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien" , is life that occurs outside of Earth and that probably did not originate from Earth. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology. Since the midth century, there has been an ongoing search for signs of extraterrestrial life. This encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life.